The present invention relates to a voltage detector for detecting the voltage developing in a selected area of an object to be measured such as an electric circuit. In particular, the present invention relates to a voltage detector of the type that detects voltage by making use of the change in light polarization that occurs in accordance with the voltage developing in a selected area of an object to be measured.
Various voltage detectors have been used to detect the voltage developing in a selected area of objects to be measured such as electric circuits. Conventional voltage detectors are roughly divided into two types: in one type, the probe is brought into contact with a selected area of an object to be measured and the voltage developing in that area is detected; and in the other type, the probe does not make contact with a selected area of an object to be measured and instead an electron beam is launched into that area and the voltage developing in it is detected.
Voltage changes rapidly in fine-line portions of objects such as integrated circuits that are small and complicated in structure, and a strong need exists in the art for detecting such rapidly changing voltage with high precision without affecting the fine-line portions. However, this need has not been fully met by the prior art voltage detectors. With devices of the type that detects voltage by bringing the probe into contact with a selected area of an object to be measured, it is difficult to attain direct contact between the probe and a fine-line portion of the object of interest such as an integrated circuit. Even if this is successfully done, it has been difficult to correctly analyze the operation of the integrated circuit solely on the basis of the voltage information picked up by the probe. A further problem involved is that contact by the probe can cause a change in the operation of the integrated circuit. Voltage detectors of the type that employs an electron beam has the advantage that they are capable of voltage detection without bringing the probe into contact with an object to be measured. However, the area to be measured with such voltage detectors has to be placed in vacuum and its surface must be exposed to that. In addition, the area to be measured is prone to be damaged by electron beams.
The prior art voltage detectors have a common problem in that they are unable to operate quickly enough to follow rapid changes in voltage and hence fail to achieve precise detection of voltages that change rapidly as in integrated circuits.
With a view to solving these problems, it has been proposed by two of the present inventors (Japanese Patent application No. 137317/1987 filed on May 30, 1987) that voltage be detected by making use of the polarization of a light beam that changes with the voltage developing in a selected area of an object to be measured.
A voltage detector operating on this principle is schematically shown in FIG. 5. The detector generally indicated by 50 is composed of the following components: an optical probe 52; a CW (Continuous-Wave) light source 53 typically in the form of a laser diode; an optical fiber 51 for guiding a light beam from the CW light source 53 into an optical probe 52 via a condenser lens 60; an optical fiber 92 for guiding reference light from the optical probe 52 into a photoelectric converter 55 via a collimator 90; an optical fiber 93 for guiding output light from the optical probe 52 into a photoelectric converter 58 via a collimator 91; and a comparator circuit 61 for comparing the electric signals from the photoelectric converters 55 and 58.
The optical probe 52 is filled with an electro-optic material 62 such as an optically uniaxial crystal of lithium tantalate (LiTaO.sub.3). The tip 63 of the electro-optic material 62 is worked into a frustoconical shape. The optical probe 52 is surrounded with a conductive electrode 64 and has at its tip 63 a coating of reflecting mirror 65 in the form of a thin metal film.
The optical probe 52 further includes the following components: a collimator 94; condenser lenses 95 and 96; a polarizer 54 for selectively extracting a light beam having a predetermined polarized component from the light beam passing through the collimator 94; and a beam splitter 56 that splits the extracted light beam from the polarizer 54 into reference light and input light to be launched into the electro-optic material 62 and which allows the output light emerging from the electro-optic material 62 to be directed into an analyzer 57. The reference light is passed through the condenser lens 95 and thence launched into the optical fiber 92, whereas the output light emerging from the electro-optic material 62 is passed through the condenser lens 96 and thence launched into the optical fiber 93.
Voltage detection with the system shown in FIG. 5 starts with connecting the conductive electrode 64 on the circumference of the optical probe 52 to a predetermined potential, say, the ground potential. Then, the tip 63 of the probe 52 is brought close to the object to be measured such as an integrated circuit (not shown), whereupon a change occurs in the refractive index of the tip 63 of the electro-optic material 62 in the probe 52. Stated more specifically, the difference between refractive indices for an ordinary ray and an extraordinary ray in a plane perpendicular to the light-traveling direction will change in the optically uniaxial crystal.
The light beam issuing from the light source 53 passes through the condenser lens 60 and is guided through the optical fiber 51 to be directed into the collimator 94 in the optical probe 52. The beam is polarized by the polarizer 54 and a predetermined polarized component having intensity I is launched into the electro-optic material 62 in the optical probe 52 via the beam splitter 56. Each of the reference light and the input light, which are produced by passage through the beam splitter 56, has an intensity of I/2. As already mentioned, the refractive index of the tip 63 of the electro-optic material 62 varies with the voltage on the object being measured, so the input light launched into the electro-optic material 62 will experience a change in the state of its polarization at the tip 63 in accordance with the change in the refractive index of the latter. The input light is then reflected from the reflecting mirror 65 and makes a return trip through the electro-optic material 62, from which it emerges and travels back to the beam splitter 56. If the length of the tip 63 of the electro-optic material 62 is written as the state of polarization of input light launched into that material will change in proportion to the difference between refractive indices for an ordinary ray and an extraordinary ray and to the length 2l as well. The output light sent back into the beam splitter 56 is thence directed into the analyzer 57. The intensity of the output light entering the analyzer 57 has been decreased to I/4 as a result of splitting with the beam splitter 56. If the analyzer 57 is designed in such a way as to transmit only a light beam having a polarized component perpendicular to that extracted by the polarizer 54, the intensity of output light that is fed into the analyzer 57 after experiencing a change in the state of its polarization is changed from I/4 to (I/4)sin.sup.2 [(.pi./ 2)V/V.sub.0 ]in the analyzer 57 before it is further fed into the photoelectric converter 58. In the formula expressing the intensity of output light emerging from the analyzer 57, V is the voltage developing in the object to be measured, and V.sub.0 is a half-wave voltage.
In the comparator circuit 61, the intensity of reference light produced from the photoelectric converter 55, or I/2, is compared with the intensity of output light produced from the other photoelectric converter 58, or (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ].
The intensity of output light, or (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ], will vary with the change in the refractive index of the tip 63 of the electro-optic material 62 that occurs as a result of the change in voltage. Therefore, this intensity can be used as a basis for detecting the voltage developing in a selected area of the object to be measured, say, an integrated circuit.
As described above, in using the voltage detector 50 shown in FIG. 5, the tip 63 of the optical probe 52 is brought close to the object to be measured and the resulting change in the refractive index of the tip 63 of the electro-optic material 62 is used as a basis for detecting the voltage developing in a selected area of the object of interest. Therefore, the voltage developing in fine-line portions of a small and complicated object such as an integrated circuit which are difficult to be contacted by a probe or which cannot be contacted by the same without affecting the voltage being measured can be effectively detected by the detector 50 without bringing the optical probe 52 into contact with such fine-line portions. If desired, a pulsive light source such as a laser diode that produces light pulses of a very small pulse width may be used as a light source to ensure that rapid changes in the voltage on the object to be measured are sampled at extremely short time intervals. Rapid changes in the voltage on the object of interest can be measured with a very high time resolution by using a CW light source and a quick-response detector such as a streak camera. Either method is capable of precision detection of rapid changes in voltage.
The voltage detector 50 shown in FIG. 5, however, has a problem that results from the fact that the conductive electrode 64 is provided on the circumference of the optical probe 52 and that this electrode is connected to a certain potential, say, the ground potential. As shown in FIG. 6, the lines of electric force ELN that are produced by the voltage developing in a selected area of the object 6, namely, difference between the potential induced in the thin metal film 65 and the ground potential to which the conductive electrode 64 is connected, do not run through the electro-optic material 62 in a direction parallel to the center line A--A of the optical probe 52 and instead are curved particularly significantly in the vicinity of the conductive electrode 64. As a result, the change that occurs in the refractive index of the electro-optic material 62 on account of the voltage developing in a selected area of the object 6 is not uniform throughout the material and the state of polarization of a light beam passing through said material will not change in a manner that is in exact correspondence to the change in the voltage developing in a selected area of the object 6, thereby making it practically impossible to accomplish correct detection of the voltage to be measured.